US4681671A - Low temperature alumina electrolysis - Google Patents

Low temperature alumina electrolysis Download PDF

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Publication number
US4681671A
US4681671A US06/829,435 US82943586A US4681671A US 4681671 A US4681671 A US 4681671A US 82943586 A US82943586 A US 82943586A US 4681671 A US4681671 A US 4681671A
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electrolyte
alumina
cell
electrolysis
anode
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US06/829,435
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English (en)
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Jean-Jacques Duruz
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Moltech Invent SA
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Eltech Systems Corp
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Assigned to ELTECH SYSTEMS CORPORATION, A CORP. OF DE. reassignment ELTECH SYSTEMS CORPORATION, A CORP. OF DE. ASSIGNMENT OF ASSIGNORS INTEREST. Assignors: DURUZ, JEAN-JACQUES
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Assigned to MOLTECH INVENT S.A.,, 2320 LUXEMBOURG reassignment MOLTECH INVENT S.A.,, 2320 LUXEMBOURG ASSIGNMENT OF ASSIGNORS INTEREST. Assignors: ELTECH SYSTEMS CORPORATION
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    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
    • C25C3/12—Anodes
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/08—Cell construction, e.g. bottoms, walls, cathodes
    • C—CHEMISTRY; METALLURGY
    • C25—ELECTROLYTIC OR ELECTROPHORETIC PROCESSES; APPARATUS THEREFOR
    • C25C—PROCESSES FOR THE ELECTROLYTIC PRODUCTION, RECOVERY OR REFINING OF METALS; APPARATUS THEREFOR
    • C25C3/00—Electrolytic production, recovery or refining of metals by electrolysis of melts
    • C25C3/06—Electrolytic production, recovery or refining of metals by electrolysis of melts of aluminium
    • C25C3/18—Electrolytes

Definitions

  • the invention relates to a method of producing aluminum by electrolysis of alumina dissolved in a molten fluoride electrolyte in an aluminum reduction cell, particularly at temperatures between 680°-900° C.
  • cryolite crusts on the cathode was caused by depletion of aluminum containing ions at the cathode and a consequent shift in the bath composition at the cathode interface to high NaF content.
  • the decrease in AlF 3 content need be only 2% at 860° C. with a bath weight ratio of 0.8 before cryolite will precipitate at the cathode.
  • the local decrease in AlF 3 has to be greater than 7% before cryolite precipitates.
  • the proposed method should, in particular, solve the problems related to low alumina solubility and solution rate in molten cryolite at these low temperatures.
  • the above object is met by a method of producing aluminum by electrolysis of alumina dissolved in a molten fluoride electrolyte in an aluminum reduction cell, at a temperature below 900° C., characterized by effecting steady-state electrolysis using an oxygen-evolving anode at an anodic current density which is at or below a threshold value corresponding to the maximum transport rate of oxide ions in the electrolyte and at which oxide ions are discharged preferentially to fluoride ions, the electrolyte circulating between an electrolysis zone wherein the electrolyte is depleted of alumina and an enrichment zone where the electrolyte is enriched with alumina.
  • the invention is based on the insight that oxide ions in low concentrations, as in the case of low temperature melts, could be discharged efficiently provided the anode current density does not exceed the above threshold current density. Exceeding this value would lead to the discharge of fluoride ions which has been observed in experiments using carbon anodes.
  • the latter In order to carry out a stable electrolysis under the given temperature conditions and the corresponding low solubility of alumina in the low temperature electrolyte the latter is circulated from the electrolysis zone to an enrichment zone and back, to facilitate and eventually speed up the solution rate of alumina.
  • the temperature of the electrolyte may be in the range of 680° C.-900° C., in particular between 700° C.-750° C.
  • the above circulation is provided for two purposes, one to prevent blockage of the cathode through build-up of solid Na 3 AlF 6 at its surface and the other to insure efficient transport of alumina to the anode surface.
  • the electrolyte may be kept in forced circulation along a predetermined circulation path by appropriate means such as a pump or a stirring mechanism, or it may be circulated by convection. Melt circulation near the inert anode surface could be enhanced by using the effect of oxygen gas lift.
  • the electrolyte may be circulated between the electrolysis zone and the enrichment zone disposed within the same cell compartment or the enrichment zone may be located in a saturator unit separated from the electrolysis zone confined in an electrolysis compartment.
  • Alumina feed could be either directly into the top of the cell or preferably into the saturator unit through which the alumina-exhausted electrolyte is passed.
  • This unit may operate under such conditions of temperature and hydrodynamic flow that alumina dissolves at an appropriate rate.
  • the temperature of the melt in the saturator unit may be higher than the operating temperature in the electrolysis compartment or in the electrolysis zone.
  • a heat exchange between the electrolyte leaving and entering the saturator unit may be provided.
  • the heating may be effected by any suitable means such as steam or other.
  • the electrolyte may comprise a mixture of NaF, LiF and AlF 3 , the concentration thereof being selected within a range of 0-48 w% NaF, 0-48 w% LiF and 42-63 w% AlF 3 , so long as NaF or LiF are present with AlF 3 the temperature of the electrolyte being in the range of 680°-900° C.
  • the anodic current density used in the method according to the invention may be up to 5 times lower than the one conventionally employed in Hall-Heroult cells being generally between 0.6 and A/cm 2 and the cathodic current density may be kept at conventional levels (0.6-1.2 A/cm 2 ) or lowered likewise.
  • the ratio between the anodic and cathodic current densities may be as low as 1:5, in the second case both current densities may be essentially equal.
  • the anodic current density can be in the range of 0.1-0.5 A/cm 2 .
  • the total anode surface must be increased maintaining an equivalent production capacity per unit floor surface. Therefore, the anode must have a special design such as a blade configuration or a porous reticulated structure.
  • anode having low current density characteristics together with a cathode working at normal or also at low current densities requires that such anode be dimensionally stable and of a special configuration which provides an increase of the anode electrochemical surface of at least 1.5 times up to 5 times.
  • the necessity of using an anode with a special configuration is a major reason for not using a consumable carbon anode in a low temperature electrolytic cell.
  • the anode may be composed of a metal, an alloy, a ceramic or a metal-ceramic composite, stable under the operating conditions.
  • Anode materials which satisfy such requirements are disclosed e.g. in the European Patent Application, Publication No. 0030834 and comprise mixed oxides (ferrite type), or oxyfluorides, or cermets as disclosed in the U.S. Pat. No. 4,397,729.
  • An electrolytic alumina reduction cell may contain a molten fluoride electrolyte with dissolved alumina having a temperature below 900° C., an inert oxygen-evolving anode and a cathode.
  • the anode may have an electrochemically active surface area sufficiently large to allow it to operate with an anodic current density which is at or below a threshold value corresponding to the maximum transport rate of oxide ions in the electrolyte of the above indicated low temperature and at which oxide ions are discharged preferentially to fluoride ions, the electrolyte circulating between an electrolysis zone wherein the electrolyte is depleted of alumina and an enrichment zone where the electrolyte is enriched with alumina.
  • An alumina reduction cell according to the invention may comprise an electrochemically active surface anode area up to 5 times larger than the projected area of the anode onto a horizontal plane, the surface area of the cathode may be kept at classic values or increased likewise.
  • the latter may e.g. be the case in a cell having a drained cathode configuration whereby the cathode comprises a shape following the surface of the anode in a small distance therefrom.
  • the enrichment zone of the alumina reduction cell my be embodied by a saturator unit separate from an electrolysis compartment of the cell, and the circulation of the molten electrolyte delivering alumina-depleted electrolyte from the electrolysis compartment to the saturator unit and returning electrolyte enriched with alumina from the saturator unit to the electrolysis compartment may be effected by means providing forced circulation of the molten electrolyte.
  • the electrolytic cell is preferably totally enclosed and contains no frozen electrolyte.
  • Alumina or any other melt resistant material could advantageously be used as liner for the enclosure.
  • the total surface of the cathode may be such that the cathodic current density remains at a value comparable with the one in classical Hall-Heroult cells or it may also be decreased.
  • the decrease of the cathodic current density is given by the re-dissolution of the product metal in the electrolyte and its subsequent oxidation at the anode, the dissolution rate being dependent on the cathode (or production metal) surface.
  • the re-dissolution decreases the current efficiency and is therefore a limiting factor for an increase of the cathode surface.
  • This effect is significant in Hall-Heroult cells using an aluminum pad. In a cell using a cathode from which the produced aluminum is constantly drained, however, the dependency of the re-dissolution rate from the cathode surface is less important.
  • the cathode therefore comprises preferably a configuration which allows continuous draining of the produced metal and it may be composed of a refractory hard metal (RHM) or a composite material thereof which may be disposed either horizontally or vertically.
  • RHM refractory hard metal
  • the RHM material mentioned above may e.g. comprise an oxide, boride or carbide of titanium, zirconium, hafnium, vanadium, niobium or tantalum or a mixture thereof.
  • the bath composition may be chosen according to several limiting or determining conditions, the most important ones being:
  • the bath has to be liquid at the chosen operating temperature
  • the anodic reaction must be oxygen evolution
  • melt constituents other than aluminum
  • FIG. 1 is a schematic polarization curve in low temperature Na 3 AlF 6 .AlF 3 melts.
  • FIG. 2 is a schematic diagram of an enclosed electrolysis cell and recirculation systems.
  • FIG. 1 a schematic polarization curve is illustrated with the voltage V being plotted on the horizontal and the current density CD on the vertical axis.
  • Curve L stands for "low" temperature and low oxide ion concentration. At zero voltage, no oxide ions are discharged at the anode, even though the transport of ions starts at very small voltages, but the potential is not sufficient to discharge the ions which, therefore, form a concentration barrier near the anode surface which suppresses further transport. At the voltage V o , oxide ions begin to be discharged at the anode; the discharge rate depends on the voltage, increasing rapidly between V o and V 1 . At voltages higher than V 1 the increase of the oxide ion discharge becomes smaller and shows essentially zero growth between V 1 and V 2 which is due to the saturation of the oxide ion transport caused by the maximum oxide ion mobility.
  • the current density CD o in this range corresponds to the threshold current density as defined above.
  • the range between V 1 and V 2 is the optimum operation range for the cell configuration according to the invention. An increase of the voltage beyond V 2 causes the discharge of fluoride ions to begin.
  • the diagram shows a second curve H, standing for "high" oxide ion concentration and high temperature. This second curve H shows a slope without a plateau between V 1 and V 2 , since the concentration of oxide ions is high enough and no saturation of the oxide ion transport will be reached in the given range of voltages and current densities.
  • FIG. 2 shows a schematic cross section of an aluminum production cell adapted to carry out the method according to the invention.
  • the cell comprises an electrolysis compartment 1 including a series of vertically depending blade-like anodes 2 arranged in the upper portion of the compartment 1.
  • a horizontal cathode 3 is provided at the bottom of the compartment 1.
  • the ends of the blade anodes 2 face the cathode 3 and provide the projected area of the anodes onto the horizontal cathode 3.
  • the blade anodes 2 however also have electrochemically active sides of the blades and thus have total electrochemical surface larger than such projected area.
  • the cathode 3 comprises passage holes 13 for the passage of liquid cell contents as described further below.
  • the compartment further comprises several outlets, one outlet 5 at the top of the compartment 1 for oxygen and one, 6 at the bottom for product aluminum.
  • a third outlet 7 located above the anodes 2 serves for the withdrawal of the electrolyte 4 from the compartment 1, this outlet 7 leading to a vessel which, in the following, will be referred to as saturator unit 8, in which the electrolyte is saturated with alumina, advantageously at temperatures higher than the temperature of the electrolyte in the compartment 1.
  • the saturator unit 8 comprises an inlet 9 by which the alumina and possibly other feed or replacement material may be introduced in the saturator unit.
  • a conduit 10 for the saturated electrolyte connects the saturator unit with the bottom of the cell compartment 1, extending a certain distance into the cell compartment as to penetrate a pool 11 of molten product aluminum which has been collected at the cell bottom.
  • the passage holes 13 in the cathode are provided to permit the passage of the electrolyte 4 which is circulated by means of a pump or by electromotive forces.
  • the electrolyte is circulated so as to enter the compartment 1 at the bottom, penetrate the cathode 3 by its passage holes 13, flow upwards between the anodes 2 and leave the compartment 1 depleted of alumina, by the outlet 7 to be fed into the saturator unit, wherein it is re-saturated with alumina.
  • Aluminum metal which is produced by the electrolysis flows down through the holes 13 of the cathode 3 and is collected at the bottom of the compartment 1, from where it may be withdrawn continuously or batchwise. Oxygen, being the second product of the electrolysis, is released by the outlet 5.
  • the purpose of the circulation of the electrolyte is to remove the alumina-depleted electrolyte from between the anodes, which otherwise will cause frequent anode effects, as the replenishing of the alumina concentration may not be effective otherwise in these relatively small cross sections between the anodes.
  • the illustrated cell is only a schematic sketch and does not limit the scope of the invention to this embodiment.
  • the cell design may be modified such that the cell comprises only one compartment which contains the electrolysis zone and the enrichment zone, circulation being maintained between these two zones.
  • the anodic current density is far smaller than the cathodic one, due to the fact that the total surface of the anodes is larger than that of the cathode.
  • the concept of reducing the anodic current density is realized by the cell according to FIG. 2 in a manner to maintain the production rate of aluminum per unit floor surface at the classic level, since the cathodic current density is the same as in a Hall-Heroult cell.
  • the principle of operating an aluminum cell at low anodic current density may alternatively be realized by simply reducing the current between anode and cathode, however, the production rate of such a cell would be decreased accordingly.
  • the cell according to FIG. 2 maintains the overall current and increases the anode surface, thus maintaining the economic conditions of a classic aluminum cell.
  • Example I The experiment of Example I was repeated at a temperature of 760° C. and for a duration of 30 hours.
  • the anode and cathode current densities were 0.1 and 0.9 A/cm 2 respectively.
  • the cell voltage was 3.2 V and the current efficiency was 81%.

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  • Chemical & Material Sciences (AREA)
  • Engineering & Computer Science (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Electrochemistry (AREA)
  • Materials Engineering (AREA)
  • Metallurgy (AREA)
  • Organic Chemistry (AREA)
  • Electrolytic Production Of Metals (AREA)
US06/829,435 1985-02-18 1986-02-13 Low temperature alumina electrolysis Expired - Lifetime US4681671A (en)

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Application Number Priority Date Filing Date Title
EP85810063 1985-02-18
AT810063/85 1985-02-18

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US4681671A true US4681671A (en) 1987-07-21

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US (1) US4681671A (fr)
EP (1) EP0192602B1 (fr)
JP (1) JPH0653953B2 (fr)
AU (1) AU573069B2 (fr)
BR (1) BR8600681A (fr)
CA (1) CA1276906C (fr)
DE (1) DE3687072T2 (fr)
NO (1) NO176189C (fr)

Cited By (30)

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Publication number Priority date Publication date Assignee Title
WO1989006289A1 (fr) * 1987-12-28 1989-07-13 Aluminum Company Of America Cellule et procede electrolytiques
US4865701A (en) * 1988-08-31 1989-09-12 Beck Theodore R Electrolytic reduction of alumina
US4921584A (en) * 1987-11-03 1990-05-01 Battelle Memorial Institute Anode film formation and control
US4999092A (en) * 1988-03-29 1991-03-12 Metallurg, Inc. Transporting a liquid past a barrier
US5015343A (en) * 1987-12-28 1991-05-14 Aluminum Company Of America Electrolytic cell and process for metal reduction
US5279715A (en) * 1991-09-17 1994-01-18 Aluminum Company Of America Process and apparatus for low temperature electrolysis of oxides
US5362366A (en) * 1992-04-27 1994-11-08 Moltech Invent S.A. Anode-cathode arrangement for aluminum production cells
US5378325A (en) * 1991-09-17 1995-01-03 Aluminum Company Of America Process for low temperature electrolysis of metals in a chloride salt bath
US5498320A (en) * 1994-12-15 1996-03-12 Solv-Ex Corporation Method and apparatus for electrolytic reduction of fine-particle alumina with porous-cathode cells
US5527442A (en) * 1992-04-01 1996-06-18 Moltech Invent S.A. Refractory protective coated electroylytic cell components
US5618403A (en) * 1995-08-07 1997-04-08 Moltech Invent S.A. Maintaining protective surfaces on carbon cathodes in aluminium electrowinning cells
US5651874A (en) 1993-05-28 1997-07-29 Moltech Invent S.A. Method for production of aluminum utilizing protected carbon-containing components
US5683559A (en) * 1994-09-08 1997-11-04 Moltech Invent S.A. Cell for aluminium electrowinning employing a cathode cell bottom made of carbon blocks which have parallel channels therein
US5725744A (en) * 1992-03-24 1998-03-10 Moltech Invent S.A. Cell for the electrolysis of alumina at low temperatures
US5728466A (en) * 1995-08-07 1998-03-17 Moltech Invent S.A. Hard and abrasion resistant surfaces protecting cathode blocks of aluminium electrowinning cells
US5753163A (en) 1995-08-28 1998-05-19 Moltech. Invent S.A. Production of bodies of refractory borides
US6001236A (en) 1992-04-01 1999-12-14 Moltech Invent S.A. Application of refractory borides to protect carbon-containing components of aluminium production cells
WO2001042535A1 (fr) * 1999-12-09 2001-06-14 Moltech Invent S.A. Extraction electrolytique d'aluminium a l'aide d'anodes metalliques
WO2002088432A1 (fr) * 2001-04-27 2002-11-07 Norsk Hydro Asa Dispositif d'anode pour utilisation dans une cellule electrolytique
WO2003089686A1 (fr) * 2002-04-22 2003-10-30 Palmer Forrest M Procede et appareil de fusion d'aluminum
US20040231460A1 (en) * 2003-05-20 2004-11-25 Chun Changmin Erosion-corrosion resistant nitride cermets
US20040231459A1 (en) * 2003-05-20 2004-11-25 Chun Changmin Advanced erosion resistant carbide cermets with superior high temperature corrosion resistance
US20060137486A1 (en) * 2003-05-20 2006-06-29 Bangaru Narasimha-Rao V Advanced erosion resistant oxide cermets
US20060162555A1 (en) * 2002-10-16 2006-07-27 Norsk Hydro Asa Method for operating one or more electrolysiscells for production of aluminium
US20070006679A1 (en) * 2003-05-20 2007-01-11 Bangaru Narasimha-Rao V Advanced erosion-corrosion resistant boride cermets
US20070128066A1 (en) * 2005-12-02 2007-06-07 Chun Changmin Bimodal and multimodal dense boride cermets with superior erosion performance
US20070151415A1 (en) * 2003-05-20 2007-07-05 Chun Changmin Large particle size and bimodal advanced erosion resistant oxide cermets
US20090186211A1 (en) * 2007-11-20 2009-07-23 Chun Changmin Bimodal and multimodal dense boride cermets with low melting point binder
US20100315504A1 (en) * 2009-06-16 2010-12-16 Alcoa Inc. Systems, methods and apparatus for tapping metal electrolysis cells
CN115849419A (zh) * 2022-11-22 2023-03-28 贵州大学 一种载氟氧化铝的生产方法及生产的载氟氧化铝的应用

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US5217583A (en) * 1991-01-30 1993-06-08 University Of Cincinnati Composite electrode for electrochemical processing and method for using the same in an electrolytic process for producing metallic aluminum
DE69210038T2 (de) * 1991-11-20 1996-09-05 Moltech Invent S.A., Luxemburg/Luxembourg Zelle für die elektrolyse von tonerde,vorzugsweise bei niedrigeren temperaturen
CA2128213A1 (fr) * 1992-01-16 1993-07-22 Jainagesh A. Sekhar Element electrique chauffant, materiaux composites connexes, compose et methode de fabrication desdits produits par synthese sous matrice
US5560846A (en) * 1993-03-08 1996-10-01 Micropyretics Heaters International Robust ceramic and metal-ceramic radiant heater designs for thin heating elements and method for production
US5837632A (en) * 1993-03-08 1998-11-17 Micropyretics Heaters International, Inc. Method for eliminating porosity in micropyretically synthesized products and densified
US5320717A (en) * 1993-03-09 1994-06-14 Moltech Invent S.A. Bonding of bodies of refractory hard materials to carbonaceous supports
WO1994020650A2 (fr) * 1993-03-09 1994-09-15 Moltech Invent S.A. Cathodes traitees au carbone utilisees dans la production d'aluminium
US5374342A (en) * 1993-03-22 1994-12-20 Moltech Invent S.A. Production of carbon-based composite materials as components of aluminium production cells
US5397450A (en) * 1993-03-22 1995-03-14 Moltech Invent S.A. Carbon-based bodies in particular for use in aluminium production cells
US5938914A (en) * 1997-09-19 1999-08-17 Aluminum Company Of America Molten salt bath circulation design for an electrolytic cell
GB2372257A (en) * 1999-06-25 2002-08-21 Bambour Olubukola Omoyiola Extraction of aluminum and titanium

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Cited By (43)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US4921584A (en) * 1987-11-03 1990-05-01 Battelle Memorial Institute Anode film formation and control
WO1989006289A1 (fr) * 1987-12-28 1989-07-13 Aluminum Company Of America Cellule et procede electrolytiques
US5015343A (en) * 1987-12-28 1991-05-14 Aluminum Company Of America Electrolytic cell and process for metal reduction
US4999092A (en) * 1988-03-29 1991-03-12 Metallurg, Inc. Transporting a liquid past a barrier
US4865701A (en) * 1988-08-31 1989-09-12 Beck Theodore R Electrolytic reduction of alumina
US5279715A (en) * 1991-09-17 1994-01-18 Aluminum Company Of America Process and apparatus for low temperature electrolysis of oxides
US5378325A (en) * 1991-09-17 1995-01-03 Aluminum Company Of America Process for low temperature electrolysis of metals in a chloride salt bath
US5415742A (en) * 1991-09-17 1995-05-16 Aluminum Company Of America Process and apparatus for low temperature electrolysis of oxides
US5725744A (en) * 1992-03-24 1998-03-10 Moltech Invent S.A. Cell for the electrolysis of alumina at low temperatures
US5527442A (en) * 1992-04-01 1996-06-18 Moltech Invent S.A. Refractory protective coated electroylytic cell components
US6001236A (en) 1992-04-01 1999-12-14 Moltech Invent S.A. Application of refractory borides to protect carbon-containing components of aluminium production cells
US5362366A (en) * 1992-04-27 1994-11-08 Moltech Invent S.A. Anode-cathode arrangement for aluminum production cells
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EP0192602A1 (fr) 1986-08-27
NO176189B (no) 1994-11-07
AU573069B2 (en) 1988-05-26
JPH0653953B2 (ja) 1994-07-20
JPS61210196A (ja) 1986-09-18
NO860582L (no) 1986-08-19
DE3687072D1 (de) 1992-12-17
DE3687072T2 (de) 1993-03-18
NO176189C (no) 1995-02-15
AU5372186A (en) 1986-08-21
BR8600681A (pt) 1986-11-04
EP0192602B1 (fr) 1992-11-11
CA1276906C (fr) 1990-11-27

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